Multistage polymerisation processes are well known and widely used in the art for polymerising polypropylene. Process configurations containing at least one slurry phase polymerisation reactor and at least one gas phase polymerisation reactor are disclosed e.g. in U.S. Pat. No. 4,740,550, and further e.g. in WO98/058975 and WO98/058976. A prepolymerisation reactor is often included in the process configuration, typically to maximise catalyst performance. The use of prepolymerisation also avoids overheating the catalyst particles. Prepolymerisation also helps to ensure a more even polymerisation on the catalyst particles reducing the probability of creating fines in later reaction steps.
Single site catalysts have been used to manufacture polyolefins for many years. Countless academic and patent publications describe the use of these catalysts in olefin polymerisation. One big group of single site catalysts are metallocenes, which are nowadays used industrially and polyethylenes and polypropylenes in particular are often produced using cyclopentadienyl based catalyst systems with different substitution patterns.
Single site catalysts are used in propylene polymerisation in order to achieve some desired polymer properties. However, there are some problems in using single site catalysts on industrial scale in multistage polymerisation configurations. Thus, there is room for improving the process and catalyst behaviour in the process.
In slurry and gas phase processes alike, catalysts need to be made of solid, uniform particles of appropriate particle size, morphology and mechanical stability to avoid reactor fouling, sheeting and line plugging.
The use therefore of a catalyst support is common place. Metallocenes are conventionally supported on a carrier such as silica. The use however of supported catalysts is associated with problems such as silica residues in the final product. Further, there is still room for improved activity, and improved polymer particle formation.
In WO03/051934, the inventors proposed an alternative form of catalyst which is provided in solid form but does not require a conventional external carrier material such as silica. The invention is based on the finding that a homogeneous catalyst system containing an organometallic compound of a transition metal can be converted, in a controlled way, to solid, uniform catalyst particles by first forming a liquid/liquid emulsion system, which comprises as the dispersed phase, said solution of the homogeneous catalyst system, and as the continuous phase a solvent immiscible therewith, and then solidifying said dispersed droplets to form solid particles comprising the said catalyst.
The invention described in WO03/051934 enabled the formation of solid spherical catalyst particles of said organotransition metal catalyst without using e.g. external porous carrier particles, such as silica, normally required in the art. Thus, problems relating to catalyst silica residues can be solved by this type of catalyst. Further, it could be seen that catalyst particles having improved morphology, will give, due to the replica effect, polymer particles having improved morphology as well.
Some multistage polymerisation utilise a slurry phase followed by a gas phase set up. One of the possible limitations of polymerization catalysts in general, and of metallocene-based catalysts in particular, is that when the catalyst has a high activity in slurry, e.g. bulk, the activity in gas phase is often low. This makes it difficult to achieve a low bulk-to-gas phase ratio of the produced material (the so-called bulk/GP split).
To be relevant for industrial polypropylene production, a single site catalyst must have good performance under all polymerisation conditions, in particular in conditions, where polymerisation temperature is at least 60° C., and in all actual polymerisation reactors of the multistage process including both liquid (ideally bulk slurry) and gas phase reactors. The present invention tries to address this issue.
Especially in industrial production of heterophasic copolymers in a three stage polymerisation a catalyst must have, inter alia, a long enough lifetime to have still acceptable activity in the third reactor, in which the rubber phase is produced. One of the possible limitations of polymerization catalysts in general, and of metallocene-based catalysts in particular, is that when the catalyst has a high activity in bulk and in the first gas phase (GPR1) reactors, the activity in the second gas phase reactor (GPR2) is often low, not allowing attainment of a high GPR2- to -bulk+GPR1 ratio of the produced material (the so-called rubber split). Here therefore, strong (initial) activity in the bulk step can lead to faster catalyst deactivation, in turn leading to a poorly active catalyst in the second gas phase reactor.
The present inventors have now found a new class of olefin polymerisation catalysts, which are able to solve the problems disclosed above. In particular, the invention combines the use of boron based and aluminoxane cocatalysts in solid catalysts not containing any external support material, essentially prepared using the basic principles of WO03/05194.
The invention provides a solid catalyst material, where no silica support material is used and which exhibits remarkable increase in activity in the gas phase in a slurry-gas phase-gas phase polymerisation cascade. This process also avoids any problems relating to the use of the conventionally supported catalysts, such as silica supported catalysts without prejudicing activity and productivity.
Whilst both boron based and aluminoxane cocatalysts are well known in the art, they are typically used as alternatives. However, it is also known to use boron activators together with aluminoxanes in some circumstances.
EP-A-0574258 discloses use of boron compounds together with aluminoxanes in single site catalysts. The catalysts are homogeneous, however, and they are used in homogeneous polymerisation where activity increase could be observed.
In J Macromol. Chem Phys, 199, 2409-2416 (1998), there is a disclosure of the use of constrained geometry metallocene type catalysts with both a methyl aluminoxane and trispentafluorophenyl boron activator. In the context of solution phase polyethylene polymerisation, the blend was found to increase catalyst activity.
In the literature, there are also other similar observations, that homogeneous catalyst activity (solution phase polymerisation) was improved by using boron modification, but when heterogeneous catalysis was tried, i.e. when catalysts were supported on silica, activity was lower than that achieved using MAO activators alone.
However, WO1998/040418 discloses that when specific types of boron-compounds, in particular alkyl or aryl boronic acids (RB(OR′)2) or cyclic boron compounds, boroxanes, are used with silica supported metallocene catalysts in combination with aluminoxanes, higher activity was seen for ethylene-butene polymerisation.
US2011294972 discloses the use of catalysts of specific transition metal complexes comprising mono-anionic, bidentate triazole ligands in combination with MAO and borate type activators supported on silica in ethylene-butene polymerisation.
The present inventors have surprisingly found that the use of both boron based cocatalysts, especially borates, and aluminoxane cocatalysts in combination in a solid, but unsupported, metallocene catalyst, allows the formation of a catalyst which address the issue of the slurry bulk to gas phase split in the context of a heterophasic propylene copolymer.
By using the modified catalysts of the present invention, a very high activity can be obtained even in the second gas phase step, much higher than the activity of the similar catalysts without borate modification. The advantage of having high activity in second gas phase is not only in the higher overall productivity of the process, but also in the achievable range of polymer properties: for example a higher gas phase split enables the production of polypropylenes with broader molecular weight distribution. In the context of a heterophasic propylene copolymer, the control of the gas phase split allows manipulation of the xylene soluble content of the polymer. Further, an increase in polymer melt temperature Tm is achieved using the process of the invention.